1. Field of the Invention
Technical Field of the Invention
The present invention relates to a device for purifying an exhaust gas of an engine.
2. Description of the Related Art
If an air-fuel ratio of an air-fuel mixture in a combustion chamber of each cylinder is referred to as an engine air-fuel ratio, there is known an exhaust gas purifying device for an internal combustion engine with multi-cylinders, in which a three-way (TW) catalyst is arranged in the exhaust gas passage, and the engine air-fuel ratio is controlled to be stoichiometric or rich with respect to the stoichiometric air-fuel ratio. If the engine air-fuel ratio is made lean with respect to the stoichiometric air-fuel ratio, the TW catalyst does not purify nitrogen oxides NO.sub.X in the exhaust gas sufficiently, and the NO.sub.X is emitted to the ambient air. Accordingly, the exhaust gas purifying device mentioned above makes the engine air-fuel ratio to be stoichiometric or rich, to thereby purify NO.sub.X, in the TW catalyst, as much as possible.
On the other hand, it is desirable to make the engine air-fuel ratio as lean as possible, to make a fuel consumption rate lower. However, if the engine air-fuel ratio is made lean, the above-mentioned exhaust gas purifying device cannot purify NO.sub.X sufficiently. To solve this problem, if a ratio of the total amount of air fed into the intake passage, the combustion chamber, and the exhaust passage upstream of a certain position in the exhaust passage to the total amount of fuel fed into the intake passage, the combustion chamber, and the exhaust passage upstream of the above-mentioned position is referred to as an exhaust gas air-fuel ratio of the exhaust gas flowing through the certain position, Japanese Unexamined Patent Publication No. 4-365920 discloses an exhaust gas purifying device for an internal combustion engine with multi-cylinders, the cylinders being divided into first and second cylinder groups, the first and second cylinder groups being respectively connected to first and second exhaust passage. In the device, the TW catalyst is arranged in the first exhaust passage, the TW catalyst synthesizing NH.sub.3 from NO.sub.X in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and an exhaust gas purifying catalyst for purifying the inflowing NO.sub.X and the inflowing NH.sub.3 is arranged in an interconnecting passage interconnecting the first exhaust passage downstream of the TW catalyst and the second exhaust passage. The first cylinder group performs a rich operation in which the engine air-fuel ratio is made rich, and the second cylinder group performs a lean operation in which the engine air-fuel ratio is made lean. In the device, the fuel consumption rate is reduced by increasing the number of the cylinders in which the lean operation is to be performed, while NO.sub.X synthesized in the cylinders in which the lean operation is performed is purified in the exhaust gas purifying catalyst as much as possible, by using NH.sub.3 which is synthesized by introducing the exhaust gas of the cylinders in which the rich operation is performed to the TW catalyst.
However, if the first cylinder group performs the rich operation and the second cylinder group performs the lean operation, as mentioned above, a fluctuation in the output torque will increase, because the first and second cylinder groups have a common output shaft. Especially, if the internal combustion engine is adapted for a vehicle such as an automobile, such an increased fluctuation may make the driver uncomfortable.
If the output torques of the first and second cylinder groups are made substantially same, the torque fluctuation can be reduced. However, to make the output torque of the first cylinder group in which the rich operation is performed is made to be substantially the same as that of the second cylinder group in which the lean operation is performed, it is necessary to perform a complicated control on, for example, the fuel injection amount or the ignition timing, in each cylinder group.
In order to solve the above problems, the Japanese Unexamined Patent Publication No. 8-4522 discloses an exhaust gas purifying device for an internal combustion engine, in which the first and the second cylinder groups perform the lean operation, and an additional internal combustion engine performing the rich operation is provided, and the exhaust gas of the additional engine is fed into the first exhaust passage upstream of the TW catalyst, to thereby make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst rich. In this device, the additional engine performs the rich operation to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the TW catalyst rich. Thus, the fluctuation in the output torque in the first and the second cylinder groups is suppressed. Note that an output of the additional engine is used to, for example, drive an auxiliary machine such as an air-conditioner.
However, an amount of NO.sub.X flowing into the exhaust gas purifying catalyst depends on an operating condition of the multi-cylinder engine, and an amount of NH.sub.3 flowing into the exhaust gas purifying catalyst depends on an operating condition of the additional engine. Therefore, when the operating condition of the multi-cylinder engine or of the additional engine changes, the NH.sub.3 amount may become excessive to or be short with respect to the NO.sub.X amount flowing into the exhaust gas purifying catalyst. Namely, when the NH.sub.3 amount becomes excessive to the NO.sub.X amount in the exhaust gas purifying catalyst, the NH.sub.3 may be discharged from the exhaust gas purifying catalyst without being purified. When the NH.sub.3 amount becomes short with respect to the NO.sub.X amount, the NO.sub.X may be discharged from the exhaust gas purifying catalyst without being purified.